There are commonly known methods for reducing light reflection on the surface of an optical element by forming a film of a material with a low refractive index, i.e., a low-refractive-index film, on the surface of the optical element. A typical method is dry deposition, in which a low-refractive-index material is deposited in a vacuum by, for example, sputtering or evaporation. Another method is wet deposition, in which low-refractive-index particles are synthesized in a liquid phase, are mixed with a coating solution, and are deposited by, for example, dip coating or spin coating. The latter method is more advantageous in terms of the cost of deposition per low-refractive-index film, and many reports have been available.
An example of low-refractive-index particles for use in wet deposition is hollow particles such as hollow silica particles. There are reports on methods of producing hollow particles by preparing core-shell particles and removing the core therefrom. For example, PTL 1 discloses a method including forming inorganic particles such as aluminum oxide or calcium carbonate particles as a core, synthesizing silica on the core to prepare core-shell particles, and removing the core therefrom by ionization. PTL 2 discloses a method including forming organic polymer particles as a core, synthesizing silica on the core to prepare core-shell particles, and removing the core therefrom by baking.